Centralizing downhole tools are employed in both rotary drilling and in fluid motor drilling applications to maintain a uniform well borehole size and to control the direction of hole development. Downhole tools used in drilling are preferably selected with diameters that closely approximate the diameter of the hole within which the tools are to be employed. Tools that automatically change their diametric dimensions to adapt to varying hole sizes have also been used for this purpose.
An example of a tool that automatically extends stabilizing arms to adapt the tool to a variable diameter borehole is described in Applicant's International application Ser. No. PCT/BE93/00073 filed Dec. 3, 1993. The cited prior art tool employs the drilling fluid pressure to drive several laterally movable pistons against contact arms that pivot out from the tool into engagement with the surrounding borehole wall. A central tubular piston converts the drilling fluid pressure to a hydraulic pressure that acts against the lateral pistons. A timing wheel connects the lateral pistons to each other to coordinate their lateral movement. The outward movement of the lateral pistons compresses individual springs positioned about each lateral piston to assist in retracting the pistons and connected contact arms when the drilling fluid pressure is reduced.
Retraction of the pistons and engaged contact arms in the prior art device is dependent upon the spring forces acting on the individual lateral pistons and on the pressure differential acting across the pistons. In some applications, these spring and pressure induced forces may not be adequate to ensure full retraction of the contact arms.